The invention concerns a method for automatically controlling an internal combustion engine, in which, in a first controller mode, a charge air temperature controller is set as dominant for a map-controlled thermostatic valve, and, in a second controller mode, a coolant temperature limit controller is set as dominant for the map-controlled thermostatic valve. Furthermore, upstream of a recooler, a coolant flow is divided into a recooler coolant flow and a bypass coolant flow as a function of the position of the map-controlled thermostatic valve, and downstream of the recooler, the temperature of the charge air cooler coolant flow is determined from the combined fractions of the recooler coolant flow and the bypass coolant flow to control the charge air temperature or to control the coolant temperature.
A cooling circulation system of an internal combustion engine is known from the journal Schiff & Hafen/Kommandobrücke, No. 1, 1990, pp. 49-50. The cooling circulation consists of a high-temperature circuit with a coolant pump and a low-temperature circuit that branches off from the high-temperature circuit. In the low-temperature circuit, a thermostatic valve, a recooler with a bypass line, a charge air cooler, and a lubricating oil heat exchanger are arranged in series. The coolant flow in the low-temperature circuit diverted from the high-temperature circuit is divided by the position of the thermostatic valve into a recooler coolant flow, which flows through the recooler, and a bypass coolant flow. Downstream of the recooler, the two coolant flows are brought together again and fed to the charge air cooler as the charge air cooler coolant flow. The thermostatic valve thus determines the temperature of the charge air cooler coolant flow via the distribution of the coolant flow. The temperature of the charge air cooler coolant flow in turn defines, via the temperature difference from the charge air temperature, the amount of heat extracted from the charge air in the charge air cooler. For example, when the internal combustion engine is under full load, the thermostatic valve is completely open, so that the entire coolant flow of the low-temperature circuit flows through the recooler, and therefore the greatest possible amount of thermal energy is extracted from the charge air. By contrast, when the internal combustion engine is idling, the thermostatic valve is completely closed, so that the entire coolant flow of the low-temperature circuit flows through the bypass line, and very little thermal energy is extracted from the charge air. The switching state of the thermostatic valve is determined by a thermostatic operating element, for example, an expanding material element, which expands with increasing temperature of the coolant flow, so that the thermostatic valve opens, or contracts with decreasing temperature of the coolant flow, so that the thermostatic valve closes under spring tension. Due to the design of the thermostatic valve, automatic control of the charge air temperature is still not possible at all operating points.
DE 201 22 420 U1 discloses an electrically heated thermostatic valve in which the characteristic curve of the thermostatic operating element can be shifted by the electric control of the heating element. For example, when an internal combustion engine is cold, and a large load is demanded, the coolant flow can be influenced by the bypass line at an earlier time than would be possible by the thermostatic operating element alone. In the remainder of the text, a thermostatic valve of this type will be referred to as a map-controlled thermostatic valve. DE 102 23 686 A1 discloses a corresponding method for controlling this thermostatic valve. It describes two-position control with input control of the operating element. However, this method does not offer significant improvement with respect to the automatic control of the charge air temperature in the coolant circulation described above.
The unprepublished German patent application with the official file number DE 10 2007 047 089.6 describes a method for automatically controlling the charge air temperature over the entire operating range of the internal combustion engine. The charge air temperature is automatically controlled in its own closed-loop control system with a charge air temperature controller, which acts on the map-controlled thermostatic valve in the low-temperature circuit described above. In this closed-loop control system, the charge air temperature represents the controlled variable, and the bypass coolant flow to be set represents the correcting variable. The controlled system comprises the map-controlled thermostatic valve, the recooler with bypass line, and the charge air cooler.
If the charge air temperature is automatically controlled, the coolant temperature cannot also be automatically controlled to a predetermined value, but rather the coolant temperature in the low-temperature circuit and thus in the high-temperature circuit as well merely develops as a result. In the event of a rapid load application, for example, a rapid run-up, excessive temperature elevation can occur, especially in the charge air being cooled, as a consequence of excessively hot coolant. This causes increased wear of the parts of the internal combustion engine and can bring about a reduced service life.